The death toll in the South Korean ferry disaster rose to 152 on Wednesday, as the investigation into the cause of the accident widened with raids on the offices of the vessel's operator.

Prosecutors raided 10 firms and other organisations connected to the ferry operator, Chonghaejin Marine, a week after the Sewol capsized off the country's south-west coast.

With more than 150 bodies thought to be still trapped inside the ship, the death toll is expected to exceed 300. Of the 476 passengers aboard the ship when it began to list and sink last Wednesday morning, only 174 were rescued.

Most of the victims are teenagers who were among a party of 339 pupils and teachers from a high school near Seoul. They had been on their way to a pre-exam break on the southern resort island of Jeju when the accident happened.

On Wednesday prosecutors raided the home of Yoo Byung-un, the head of a family that runs the Sewol's parent company, as well as a church he is believed to have a stake in.

Chonghaejin Marine's finances came under scrutiny after it emerged that Yoo had spent four years in prison for fraud during the early 1990s. Kim Hoe-jong, a prosecutor, said the raid was part of an investigation into "overall corruption in management".

The cause of South Korea's worst maritime disaster for more than 20 years has yet to be established. Officials are still examining the possibility that the ship listed after taking a sharp right turn as it passed through a strait dotted with tiny islands in the final stages of its journey from the western port of Incheon to Jeju.

Ahn Sang-don, a senior prosecutor, said investigators were taking several possible factors into account, including wind speed and direction, ocean currents, modifications made to the ship, freight storage and the fact that the vessel turned just before it began listing.

A maritime expert who spoke to the ship's third mate said he believed the Sewol's steering gear could have caused the accident. Kim Woo-sook of Mokpo National Maritime University said Park Han-gyeol told him she had ordered the helmsman to execute a five-degree turn, but that the steering gear turned too far and could not be returned to the correct position.

Tracking data show the ship made a 45-degree turn, a rapid change of course that could have upset the ship's balance and caused it to list, according to officials at the oceans and fisheries ministry.

The Sewol's captain, Lee Joon-seok, and six crew members have been arrested, while another two crew members have been taken into custody.

Amid stories of acts of bravery by some crew members, Lee and other senior officers have come under fire for their response to the emergency.

He was not on the bridge when the Sewol started listing, and had handed the controls to Park, an inexperienced third mate who was navigating the stretch of water for the first time.

Lee initially told passengers to stay put, even though the ship had already started to list severely. There are reports that his belated order to evacuate was not relayed over the public address system.

Lee and some of his crew have yet to explain why they abandoned the sinking ship in lifeboats while hundreds of passengers remained on board. Twenty-two of the 29 members of the ferry's crew survived.

The crew's actions drew fierce criticism from the South Korean president, Park Geun-hye, who described their actions as "tantamount to murder".

More than 70 executives and other people connected with Chonghaejin Marine and its affiliates are subject to 30-day travel bans while they are investigated on possible charges ranging from criminal negligence to embezzlement.

Chonghaejin Marine shareholders issued a statement in which they said they felt "infinite sadness and responsibility".

"We will humbly accept all responsibility for this accident and we will not hesitate to do anything to console the pain of victims and grieving families even a little bit," the statement said.

The task of locating and retrieving bodies from the sunken vessel is becoming more difficult now that divers have extended the search to individual cabins.

"The lounge is one big open space, so once in it we got our search done straight away. But in the case of the cabins, we will have to break down the walls in between because they are all compartments," said Koh Myung-seok, a rescue task force spokesman.

As the funerals for another 25 teenagers were held in their hometown of Ansan, near Seoul, South Korean media reported that many of the victims whose bodies have been retrieved sustained broken fingers, apparently caused by frantic attempts to escape along vertical walls and ceilings.

Officials said efforts to locate the remaining bodies would continue. More than 200 boats, 34 aircraft and about 500 personnel are involved in the operation.